was an accomplance in, took part in, played a role in, was involved in the kidnapping of her own son.

she was involved in kidnapping her own son

This can also be translated almost word-for-word:
She had a hand in the kidnapping of her own son

@tj: I like your word-for-word one, although I had come up with 'was the architect behind the kidnapping' in the meantime

Thank you all

@Selkie: Ein Vertipper Deinerseits oder ein Wort, was in Leo sollte?
Komplize lt. Leo: accomplice, aber nicht accomplance